1. Field of the Invention
The present invention generally relates to an apparatus for generating pseudo-noise (PN) code states in a mobile communication system, and more particularly, to an apparatus and a method for generating time-shifted PN states.
Therefore, the present invention, being capable of generating the current PN states and the previous PN states, is applicable to a direct sequence spread spectrum (DS-SS) system or a frequency hopping spread spectrum (FH-SS) system.
2. Description of the Related Art
Although the technology for commercializing the code division multiple access (CDMA) digital mobile telephone system has been developed very recently, the CDMA theory was already established back in 1950's, and it has been used in military communication. Especially, wiretapping prevention is very important in military communication. Among others, the spread spectrum, a basis of the CDMA technology, became a big help to the military to shun off any form of wiretapping.
During the meantime, Qualcomm, Inc., of the United States found the way to apply the frequency spread spectrum to the CDMA technology that was mainly used in military communication. Thanks to the newly developed CDMA technology, a number of subscribers are able to share time and frequency, transceiving signals one another.
In other words, every subscriber in the CDMA is not distinguished by the frequency or time, but by a secrete code, and all subscribers share the identical frequency band. The code in the CDMA is known to terminals and base stations, and it is what is called pseudo-noise code or spread code.
The CDMA approved by TIA in 1993 follows the IS-95 regulation, and each carrier has 1.25 MHz band. In general, the CDMA uses the spread spectrum method, which means that the information for transmission is spread to a much broader band than the original signal. For example, the 9600 bps signal is spread in 1.25 Mbps, and here, the spread means applying another PN code to the original signal. The receiving end adopts the same PN code that was used in the sending end to the 1.25 Mbps spread signal to restore the 9600 bps original signal. Therefore, since the CDMA method spreads the signals to a broad band, the signals after the spread just look like a noise floor, and if the receiving end does not know the PN code applied to the spread, demodulation of the signals are not possible, making it difficult to do wiretapping at all.
As explained before, the CDMA is based on the spread spectrum communication method, which was not available to the public because it requires very complicated equipment and whose power control is very hard to do especially during the transmission, the development of advanced electronic technology made it possible to apply the CDMA to the mobile communication system as well.
In general digital communication, the data to be transmitted goes through digital modulation, like phase shift keying (PSK) or frequency shift keying (FSK) method. However, in the spread spectrum communication, a high-speed spread code is multiplied to the digital modulated signal to expand the bandwidth of the frequency to a broadband, and then the signal is transmitted. In the meantime, in the receiving side, the same high-speed spread code used in the sending side is multiplied to the signal, and a despread process which switches the bandwidth of the frequency back to a narrow band is carried out before demodulating the signal.
The currently being used CDMA method spreads signals that originally had 10 KHZ bandwidth to have 1.25 MHZ bandwidth, and these spread signals are equally spread to the broad frequency band, fainting the intensity of the signals compared to the original signals. Accordingly, to receive such spread signals, the entire frequency band has to be examined to realize the existence of the signals, which process is not that easy. That is, as the transmission signal faints, its interference to other signals becomes relatively small, increasing the reliability of security. Also, although the signal that is spread to the broad frequency band might interfere or make noise in the radio path, its influence is only partially considering that broad frequency band, and its strength is considerably weakened, so the speech quality may be greatly improved. Because of these merits, a great number of nations over the world are now using the CDMA method.
In general, the spread spectrum communication method is divided into a frequency hopping method (FH-CDMA) and a direct sequence method (DS-CDMA). At present, many are choosing the direct sequence method over the frequency hopping method.
In the DS-CDMA method, the narrow band data signals are demodulated by the broadband code signal, and each demodulated signal occupies a broad band instantaneously, separating the multipath signals and making the signals join the Rake receiver. In addition, in the DS-CDMA, every user shares the pilot channel and coherent receiving is possible. In contrast, in the FH-CDMA, the narrow band data signals are cut off to different frequency covering a broad band, and every signal occupies the narrow frequency band instantaneously. This method is again divided into fast FH and slow FH according to the number of bit every hop transmits. That is, if the number of bit one hop transmits is approximately one, it is fast FH, and if several, it is slow FH.
In the meantime, the DS-CDMA system divides a service area into small ones called ‘cell’, and assigns the same frequency band to every cell. Also, all base stations in charge of the cells are to use the PN code that is generated by the same PN code generator, and the code sequence generated here is spaced by offset, giving the uniqueness to each base station. Moreover, each terminal uses the PN code, generated based on the offset information and the unique code of the base station the terminal belongs, to send the transmission information by the band spreading. Then, the receiving end, through the band dispreading process using the same PN code, obtains the wanted information. In short, using a unique PN code for distinguishing the terminal besides the offset information of the base station, the synchronization between the terminal and the base station can be made much faster and easier.
As noted before, the PN code sequence generated by the PN code generator is used to make broadband spreading signals similar to noise. At this time, the DS-CDMA system uses the PN code sequence to spread the frequency band, while the FH-CDMA system uses the PN state as a frequency hopping pattern. Here, the PN code sequence indicates the outputted element (code value) every moment as a bit unit, and the PN state indicates the outputted element every moment as several bit unit.
Normally, in the mobile communication system, transmission delay exists between the sending side and the receiving side. Naturally, the PN state for use of the spectrum spread should have a PN state in accordance with the time shift to match the synchronization for demodulating the information on the sending side that has been received belatedly. Thus, an apparatus for generating PN states that enables to generate not only the current PN states but also the previous or future PN states according to the time shift is required.
FIG. 1 is a block diagram of a known apparatus for generating PN states.
Referring to FIG. 1, the PN state generating apparatus in the prior art includes a PN code generator 10 for generating N-bit PN code sequence, and a clock controller 20 for applying a clock signal to enable the PN code generator 10 to generate the PN code sequence.
Thusly configured time-shifted PN state in the prior art generates a clock signal from the clock controller 20 based on the offset information to apply the clock signal to the PN code generator 10, and the PN code generator 10, under the clock signal, generates a PN code sequence. At this time, by adjusting the clock signal generated in accordance with the offset information, the PN code generator 10 generates PN states. And, using a PN mask, the PN state that is outputted from the PN code generator can be converted to a time-shifted PN state.
However, in such conventional method, since the time-shifted PN state was generated by controlling the clock controller 20 according to the offset information, it took a considerable amount of time to obtain a wanted time-shifted PN state. For instance, if a user wishes to generate a time-shifted PN state corresponding to the time difference 3 for one time offset, the clock signal had to be applied three times consecutively for that one time offset.